/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.server.reactive;

import java.util.concurrent.atomic.AtomicBoolean;

import org.reactivestreams.Processor;
import org.reactivestreams.Publisher;

import reactor.core.publisher.Mono;

import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferFactory;

/**
 * Abstract base class for listener-based server responses, e.g. Servlet 3.1
 * and Undertow.
 *
 * @author Arjen Poutsma
 * @since 5.0
 */
public abstract class AbstractListenerServerHttpResponse extends AbstractServerHttpResponse {

    private final AtomicBoolean writeCalled = new AtomicBoolean();


    public AbstractListenerServerHttpResponse(DataBufferFactory dataBufferFactory) {
        super(dataBufferFactory);
    }


    @Override
    protected final Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body) {
        return writeAndFlushWithInternal(Mono.just(body));
    }

    @Override
    protected final Mono<Void> writeAndFlushWithInternal(
            Publisher<? extends Publisher<? extends DataBuffer>> body) {

        if (this.writeCalled.compareAndSet(false, true)) {
            Processor<? super Publisher<? extends DataBuffer>, Void> processor = createBodyFlushProcessor();
            return Mono.from(subscriber -> {
                body.subscribe(processor);
                processor.subscribe(subscriber);
            });
        }
        return Mono.error(new IllegalStateException(
                "writeWith() or writeAndFlushWith() has already been called"));
    }

    /**
     * Abstract template method to create a {@code Processor<Publisher<DataBuffer>, Void>}
     * that will write the response body with flushes to the underlying output. Called from
     * {@link #writeAndFlushWithInternal(Publisher)}.
     */
    protected abstract Processor<? super Publisher<? extends DataBuffer>, Void> createBodyFlushProcessor();

}
